This invention relates generally to a low-pressure mercury vapor discharge lamp having a particular phosphor coating to emit white light efficiently when excited by the ultra-violet radiation generated from the mercury vapor discharge. More particularly, the present type lamp construction is intended for general illumination at a color temperature of approximately 3500.degree. K. with satisfactory color rendition and comparable efficiency to that now achieved in deluxe-type fluorescent lamps.
The use of several luminescent materials in combination to produce a predetermined overall spectral energy distribution is well known. It is also well known to employ such material combinations as a blended mixture or as a plurality of two or more layers in which one layer generally further comprises a blend of the individual phosphors. A conventional phosphor combination now used as a blended mixture to produce white color emission at a white color point of approximately 3500.degree. K. employs a manganese-activated or antimony-activated strontium haloapatite phosphor constituent including various combinations thereof. Conventional deluxe type fluorescent lamps providing white color illumination also represent a compromise between luminous efficiency (lumen output per watt input) and color rendition since better color rendition is usually achieved with the reduction of as much as 35% or more in luminous efficiency. For example, a deluxe warm-white lamp having a 3000.degree. K. white color point and utilizing a blend mixture of a strontium haloapatite phosphor coactivated with antimony and manganese ions to provide green color emission with a strontium magnesium orthophosphate phosphor activated with tin ion to provide red color emission produces 2150 lumens in a 40T12 lamp size at a color rendering index (CRI) value of approximately 90.
In U.S. Pat. No., 4,079,287, which is assigned to the assignee of the present invention, there is disclosed an improved phosphor coating for a deluxe type fluorescent lamp which produces white color illumination at a color temperature in the range 2700.degree.-6500.degree. K. with a satisfactory color rendition and at higher emission efficiency than the aforementioned prior art deluxe type fluorescent lamps. More particularly, a binary phosphor combination is employed utilizing a strontium haloapatite phosphor with a europium-activated yttrium oxide phosphor to produce lumen values at the 40T12 lamp size up to 2900 along with CRI values up to 90 and higher. A ternery mixture utilizing said phosphor combination together with a conventional calcium haloapatite phosphor coactivated with antimony and manganese ions to provide said white color illumination is also disclosed. Said ternery mixture is said to contain 30-70% by weight of the calcium haloapatite phosphor constituent with the balance consisting of the aforementioned phosphor combination. Since the cost of the yttrium oxide phosphor constituent in this prior art phosphor combination greatly exceeds the cost of all remaining phosphor constituents, however, it becomes desirable to replace this phosphor constituent without unduly sacrificing the lamp performance.
It is a primary object of the present invention, therefore to provide a still lower cost phosphor combination usable in fluorescent lamps providing white color illumination at emission efficiencies and color rendition values acceptable for deluxe type lamps.
It is another important object of the present invention to provide a novel lower cost blend of phosphor materials producing white color emission for deluxe type fluorescent lamps.
Other objects of the present invention will become apparent to those skilled in the art upon consideration of the detailed description hereinafter provided.